Navy planes, like the F-14 and F-18, have stronger structures, as well as other things like folding wings that make them suitable for use on crowded carriers and doing much of their flying over water.
The F-4 was originally a Navy design, so it could adapt to a land based role. It would be difficult to do the reverse.
True. The Supermarine Spitfire was, as far as I know, the only successful land-based fighter converted to shipboard duty (as the Seafire).